The broad objectives of this study are to characterize the behavior and state of assembly of microtubules in the living cell, to elucidate the mechanism by which microtubules move granules during secretion, and to identify and define microtubular abnormalities leading to secretory dysfunction. Colchicine will be used as a probe (a) to study the interaction between microtubules and other components of the secretory apparatus, (b) to measure the amount and state of assembly of the tubulin-microtubule system in neuroblastoma cells, lymphocytes, and islet cells, and (c) to detect the presence of endogenous factors which may regulate the tubulin-microtubule system. Antibodies which are directed aginst tubulin and the high molecular weight microtubule-associated proteins will be used in immunofluorescent studies as well as in studies designed to clarify the role of the high molecular weight proteins in microtubule functions, particularly secretion.